1. Field of the Invention
This invention relates generally to imaging the volume of a turbid medium, together with objects embedded or suspended in such a medium; and more particularly to use of streak-lidar apparatus to monitor phenomena at an extremely broad range of scales—including detection of a tumor less than a millimeter across, in living tissue; or an underwater object in the ocean, or vehicles in fog, or a variety of other objects in turbid media.
2. Prior Art
The present invention has applications spanning a range of sizes, and is believed to integrate diverse, heretofore nonanalogous fields. For reasons to be explained in this document, these fields have not previously been linked.
These application fields include imaging of volumes of the atmosphere with aircraft moving through such volumes—over a range (and atmospheric volume) on the scale of kilometers. It also includes imaging of ocean volumes—together with submarines, sunken ships, submerged fuel drums and the like, over a field of examination that is some one to two kilometers wide and perhaps many kilometers long.
In addition these applications include medical imaging of human or animal tissue, with tumors in the tissue. The tumors may be a small fraction of a millimeter in diameter, either suspended within the living tissue or growing on human or animal organs at a remote interior surface of the tissue. Here the volumes of tissue that can be imaged range from perhaps two to twenty centimeters across.
Intermediate-scale applications include imaging of a fogged-in airport and its environs, together with the land and air vehicles and other structures in the area, or imaging of a riot zone (or battlefield) filled with tear gas or other nebulized material—together with people, vehicles and the like in that zone.
These many types of imaging have not heretofore been linked. Probably the reason for this is that prior artisans have not fully appreciated how to use lidar to obtain a direct distance-to-depth mapping in a simple natural real-time display, capable of direct volume implications.
At a medical or laboratory scale, most previous users have instead become entangled in fiber-optic encoders and other counter-productive digressions. Furthermore, most or all previous workers in lidar have failed to appreciate the critical importance of the confocal condition—though that condition is recognized in other fields. (By “confocal condition” we refer to configurations that cause emitted and reflected probe beams to lie very nearly coincident upon one another.)
An example of failure to appreciate the importance of that condition appears in U.S. Pat. No. 4,704,634 of Kato—who actually uses a pulsed, unconstrained spherical wave (or “flood beam”) as his emitted beam. Accordingly bench-scale lidar configurations have not been reasonably optimized.
At ocean-volume scale, lidar systems heretofore have not been made effective at all. In this case, in addition to the failures of recognition outlined in the preceding paragraph, previous workers have evidently overlooked the potential use of streak lidar.
U.S. Pat. No. 3,719,775 (predating the invention of the streak tube) to Takaoka, addressing terrain-imaging applications, mentions in passing the use of a vertical fan-shaped beam, carried by an aircraft with the wide dimension of the beam at right angles to the direction of motion. That configuration is not Takaoka's invention, and he teaches nothing about its effective use.
Heretofore neither Takaoka nor any other artisan has proposed use of such a fan-shaped beam, projected from aircraft—either with a streak tube, or with any other effective means of reading terrain-generated reflection.
The point of commonality in all the applications, at different scales, mentioned earlier is the magnitude of the effective turbidity on a per-unit-distance (or −volume) basis. This is the consideration that controls ability to probe and resolve turbid media with a pulsed laser and a streak tube. Thus ocean volumes while vastly greater in extent than living tissue are correspondingly lesser in turbidity.
Several techniques have evolved over the years for overcoming the problems associated with detecting targets in a light-scattering medium.
Ocean-volume scale—One technique uses a narrow beam from a pulsed laser, such as a doubled YAG, to scan the ocean. Generally, the beam transmitter and the receiver aperture, which must be quite large to collect sufficient energy, are scanned together, using scanning mirrors or other devices such as prisms.
The energy received from each pulse is detected with a photomultiplier, or similar quantum-limited device, and the resulting signal is amplified with a logarithmic-response amplifier, digitized and then processed. Because the pulses are short, typically 10 nanoseconds, the detection electronics must be very fast, digitizing at 200 MHZ or faster.
Since the pulse rate is low, the processing rates required to analyze the data from each pulse are within the state of the art. Such methods require the use of mechanical scanners that are slow and difficult to build, particularly if they are to be mounted on aircraft.
In accordance with a primary advantage of the present invention, the need for fast digitizing electronics and mechanical scanners is eliminated. (As will be seen, however, in certain of the applications outlined above, at least in principle fast electronics can be substituted for a streak tube.)
Another technique is range gating, which uses a pulsed flood beam and a number of gated image intensifiers with charge-coupled devices (CCDs). The intensifiers are gated on when the beam pulse reaches a specific depth.
Typically one gate is applied just as the pulse beam that encounters the object returns to the receiver, so that the full reflected return is obtained. A second intensifier is gated on a little later to detect the shadow of the object. The image of the target is obtained by taking the difference of the two images, which then eliminates the seawater backscatter and enhances the target signature.
Several drawbacks are associated with the range-gating technique. Range gating does not allow utilization of all, or substantially all, of the information returned from each pulse to create three-dimensional data sets.
Rather in such prior-art systems, although a volume of the medium is illuminated, by range gating only one specified layer (depth increment) of the illuminated medium is selected. Thus the signal above and below the range gate is rejected—discarded.
As will be clear, of the energy transmitted into the volume of the medium and returned toward the transceiver, only a small fraction is used. This operating arrangement constitutes a monumental waste of optical energy.
Additionally, a full-depth data set cannot be created from a single pulse. Rather, full-depth information can be obtained only by collecting many pulses, during which process the platform, aircraft or other vehicle must remain stationary. (To create a full-depth image, the number of shots required is a large multiplicity. Consideration of this fact is another way of appreciating the amount of energy wasted.)
Despite the availability of such techniques, existing lidar systems are limited by the size of the receiver optics that can be used in a scanner. Generally the light reflected from targets that are deeply positioned, or suspended in a very turbid medium, is weak.
Although large-diameter optics can aid in maximizing the amount of light collected from weak returns, the size of the optics that can be used in a scanner is restricted by the size of the moving prisms or mirrors. Such cumbersome mechanisms sometimes can be eliminated, as in selected applications of the present invention, by utilizing the motion of a vehicle—e. g., boat or aircraft—carrying the system so that the dimensions for scanning can be reduced to one.
The scanning problem, however, is still formidable and restricts the size of the apertures that can be used. Moreover, volume scanning systems are very expensive, and require considerable power and weight. Consequently, for large-scale applications the ability to install such systems in aircraft or other vehicles is restricted.
Furthermore, those systems that utilize range gating, instead of volume scanning, suffer from poor range resolution and area coverage. When a target object is at a different depth from the expected, the optical return is subtracted as well as the background, and poor performance results. Additionally, very large pulse energies are required to obtain signal-to-noise ratios sufficient for detecting objects at even moderate depths.
What has been needed heretofore is an imaging system that provides an accurate and reliable image of a suspended object, eliminates the problems associated with mirror scanning for large-scale systems, and utilizes all, or substantially all, of the information returned from each pulse to eliminate laser-energy waste.
Medical scale—Streak tubes have been demonstrated in transillumination geometries to detect the presence of small tumors in tissues (see, e. g., U.S. Pat. Nos. 5,278,403 and 5,142,372 to Alfano; and U.S. Pat. No. 5,140,463 to Yoo). The transillumination technique, however, yields only two-dimensional images and cannot determine the depth of a tumor.
Furthermore, transillumination yields only a shadow signature. Such data are subject to relatively poor detection range.
As can now be seen, in the field of the invention the prior art has failed to provide solutions to important difficulties of observing the operating environment and receiving communications.